Fetching details from the dictionary...
mouth ulcer; stomatitis
口内炎が痛くて食事がつらいです。
My mouth ulcer hurts, making it difficult to eat.
治りにくい口内炎には、この薬を塗るといいですよ。
For stubborn mouth ulcers, it's good to apply this medicine.
ストレスや睡眠不足で口内炎ができやすくなります。
Mouth ulcers tend to form easily due to stress and lack of sleep.